She's probably the child between the Princess and the Knight.... Even though the story said "son," maybe it was to cover the real gender? Liam can be the future Archie, who crossed worlds to meet with MC once again (or from another timeline?).
Anyway, this is such a great series! I hope it keeps being translated.
EDIT: READ SOME SPOILERS AND OH MY IT'S SO CONFUSING SO I'LL JUST WAIT FOR IT TO BE DRAWN IN WEBTOON HAHAHAHAHA BUT IT'S REALLY GOOD
@AlessaO I'm kind of late, but I believe it has to be a 'letter.' She did say, 'wait, does this count as a letter?' when she first sent her second message. So...perhaps, but probably not.